It is known to secure a dental attachment, such a dental bridge or other dental prosthesis, to implants which have been implanted into a patient's jaw bone. An attachment can be manufactured after the implant is placed in the jaw bone or before the implant is placed in the jaw bone.
Where the attachment (e.g. bridge) is manufactured after the implants have been installed in the patient, an impression is typically taken of the mouth to determine the locations of the implants relative to the jaw and each other. In such cases, it is possible to plan, with some certainty, the size and shape of the attachment and the attachment is designed to have very limited play with respect to the implants. Some limited adjustment can be provided by recesses in the attachment and the anchorage elements (also known as abutments) on the upper part of the implants. When the attachment is fitted to the implant, cement is applied to the internal surface of the recess of the attachment and the external surface of the abutment to overcome any play between these parts. Cement, or cement-like agents, may pose problems related to hardening. Most cement types harden very rapidly, making it difficult during the hardening process to achieve exact and permanent positions for the bridge. Moreover, cement may be difficult to handle in the mouth of a patient (for example, determining the correct amount of cement to use) and may cause soiling of the restoration site.
Where the attachment is designed and manufactured prior to the installment of the implants—for so-called ‘immediate loading’—the amount of play between the attachment and the implant can be considerably larger. The information about the expected implant positions must come from an implant planning system (such as SimPlant™ from Materialise NV, Belgium). The implant planning system allows a clinician to determine the optimal position of the implants in a computer environment showing digital information of the patient (for example CT, images). The implant plan can be used to design and manufacture the attachment and means (e.g. surgical templates such as SurgiGuide™, Materialise NV, Belgium) are used to transfer the planning to the patient. The play between the implants after installation and the attachment depends on the manufacturing tolerances of the bridge and the inaccuracies inherent to the medical imaging for implant planning and the surgical process of transferring the planning to the patient. Typically the play will range from 0 to 0.5 mm in the lateral direction (known as the mesio-distal or bucco-lingual direction), 0 to 0.5 mm in the vertical direction (known as the coronal-apical direction) and 0 to 5° in an angular direction (i.e. rotation about a vertical axis through the implant site). For example, a two-stage procedure can be used. The implants are installed and an impression taken of the post-operative situation several weeks after the surgical intervention. Using impression copings, the exact positions of the implants in the mouth are transferred to a gypsum model. Implant replicas embedded in this model allow a dental technician to manufacture a prostheses that fits exactly on the implants in the jaw of the patient. A disadvantage of this approach is that the prosthesis cannot be produced prior to installment of the implants.
There have been various proposals to provide a degree of adjustment between an implant and an attachment. International Patent Application WO 03/061512 describes a dental attachment with a longitudinally extending recessed wall. An implant has a longitudinally-extending portion which can fit within the recessed wall of the attachment and which can expand to anchor the attachment in a relative longitudinal displacement relative to the portion of the implant. The use of expandable components has the disadvantage that, in the expanded state, recesses are present between the flanges of the expanded component (for instance in the radial direction). These recesses are likely locations for bacteria to settle, possibly causing infections during later stages of treatment. Furthermore, using the expandable abutments, the loads are transferred to the implants by friction. This may cause problems of loosening of the attachment in case of clenching by the patient.
International Patent Application WO 2005/053564 describes a system to overcome lateral misalignments. An anchorage part of an attachment comprises a hole for a retaining screw and the neck of the retaining screw is smaller than the hole in the anchorage part. International Patent Application WO 92/03984 describes a device for anchoring a dental attachment to overcome angular deviations in the play between the bridge and the implants.
Each of the above solutions only permit adjustment of an attachment relative to an implant in a single direction (longitudinal, lateral or angular).